Recall that s1 represents unused labor and s2 represents unused wood. In the present solution s1 and s2 are not basic variables, so they both equal zero. This means that all the material and labor are being used to make tables and chairs, and there are no excess (slack) labor hours or board feet of material left over. Thus, if we enter s1 or s2 into the solution, then s1 or s2 no longer equals zero, and the use of labor or wood is decreased. If, for exam- ple, one unit of s1 is entered into the solution, then one unit of labor previously used is not used, and profit is reduced by $20.

Let us assume that one unit of s1 has been entered into the solution, so that we have one hour of unused labor (s1 = 1). Now let us remove this unused hour of labor from the solu- tion so that all labor is again being used. We previously noted that profit was decreased by $20 by entering one hour of unused labor; thus, it can be expected that if we take this hour back (and use it again), profit will be increased by $20. This is analogous to saying that if we could get one more hour of labor, we could increase profit by $20. Therefore, if we could